Ophthalmic lenses are conventionally used for observation of various locations within the eye by ophthalmologists. These ophthalmic lenses normally include a contact lens, that is, a lens that directly contacts the cornea of the eye, and may include an entry lens that is spaced in the anterior direction from the contact lens. The two lenses are normally joined by a housing.
Most ophthalmic lenses of the type just described have been created and designed for use as an observation tool utilized in conjunction with a slit lamp or ophthalmic microscope. While most prior lenses function reasonably well for use as an observation tool, the advent of laser microsurgery and the accompanying need to deliver a laser beam safely within the eye has created a need for ophthalmic lenses that not only provide improved images of the desired location in the eye but also have the capability to deliver laser energy to the desired location with minimum effect on other portions of the eye. One example of modern ophthalmic lens utilized in diagnosis and surgical procedures is disclosed in U.S. Pat. No. 4,728,183. That lens uses an aspheric entry lens to produce an aerial image spaced from and anterior of the entry lens. A combination of the contact lens and the aspheric lens produces an aerial image that has very high resolution, even in the peripheral area of the image. The lens also produces very little reflected or scattered light as well as a nondistorting path through which the laser beam can pass during treatment of a patient's pahtology. The lens disclosed in this patent also maintains a wide cone angle on the laser beam as it passes through the patient's cornea and crystalline lens to minimize energy absorption in those areas of the patient's eye. This lens, however, has a relatively small field of view on the order of.+-.45.degree.. A second contact lens currently in use and disclosed in U.S. Pat. No. 5,007,729 supplements that disclosed in the '183 patent. That lens has a relatively wide field of view on the order of.+-.62.5.degree.. This lens is especially useful for treatment of retinal detachment in the peripheral area of the fundus and also for other procedures such as panretinal photocoagulation.
Both of these prior art lenses, however, have magnifications less than 1.times.. The '183 lens has a magnification on the order of 0.96, while the lens disclosed in the '729 patent has a magnification on the order of 0.68. It is desirable to possess a magnifying lens that provides superior retinal clarity for detecting macular problems such as macular degeneration and diabetic retinal thickening. A magnifying lens will also facilitate the location of subtle vascular landmarks during macular photocoagulation. These fine details may be apparent angiographically but are hard to find without high magnification in the low contrast ophthalmoscopic image. It is preferable to maintain an aerial image that is flat or concave when viewed from the anterior direction, that is, concave as viewed by the ophthalmologist, since an image that is convex toward the ophthalmologist tends to be unacceptably distorted, and also degrades stereo perception. Moreover, it is preferably to maintain the aerial image less than about 45 millimeters from the patient's cornea so that the lens is useful with all standard slit lamp microscopes used by ophthalmologists. In addition, while achieving these desirable ends, it is also desirable to provide a lens that has a relatively wide binocular and stereoscopic field of view.
One such magnifying lens is disclosed in Crossman et al. entitled High Magnification Ophthalmic Lens, patent application Ser. No. 702,608 filed May 21, 1991. That lens is a three element lens that was developed after several unsuccessful efforts to develop a satisfactory two element lens. The three element lens is satisfactory for the intended purpose; it is, however, more difficult and more expensive to manufacture. In addition, the field of view is smaller than is desirable, leading to a requirement for positioning the lens on the eye at just the right angle so that the proper area of the fundus is displayed. In the interim, significant time and effort was devoted to the design of a two element lens. While one of ordinary skill might believe that the design of a two element lens is relatively straight forward, that is not actually the case. Many hundreds of person hours must be devoted to the design without any assurance of success.